This research describes the development of an experimental radiation testing environment to investigate the single event effect (SEE) susceptibility of the 486-DX4 microprocessor. SEE effects are caused by radiation particles that disrupt the logic state of an operating semiconductor, and include single event upsets (SEU) and single event latchup (SEL). The relevance of this work can be applied directly to digital devices that are used in spaceflight computer systems. The 486-DX4 is a powerful commercial microprocessor that is currently under consideration for use in several spaceflight systems. As part of its selection process, it must be rigorously tested to determine its overall reliability in the space environment, including its radiation susceptibility. The goal of this research is to experimentally test and characterize the single event effects of the 486-DX4 microprocessor using a cyclotron facility as the fault-injection source. The test philosophy is to focus on the "operational susceptibility," by executing real software and monitoring for errors while the device is under irradiation. This research encompasses both experimental and analytical techniques, and yields a characterization of the 486-DX4's behavior for different operating modes. Additionally, the test methodology can accommodate a wide range of digital devices, such as microprocessors, microcontrollers, ASICS, and memory modules, for future testing. The goals were achieved by testing with three heavy-ion species to provide different linear energy transfer rates, and a total of six microprocessor parts were tested from two different vendors. A consistent set of error modes were identified that indicate the manner in which the errors were detected in the processor. The upset cross-section curves were calculated for each error mode, and the SEU threshold and saturation levels were identified for each processor. Results show a distinct difference in the upset rate for different configurations of the on-chip cache, as well as proving that one vendor is superior to the other in terms of latchup susceptibility. Results from this testing were also used to provide a mean-time-between-failure estimate of the 486-DX4 operating in the radiation environment for the International Space Station.